1. Field of the Invention
This invention relates generally to heat pump systems, and more particularly to an apparatus and a method for controlling heat reclamation after termination of a defrost cycle, and for adjusting how long each subsequent defrost cycle operates by adjusting when to initiate each subsequent defrost, so that the defrost cycle terminates only when the coil is free from frost.
2. Prior Art
Air conditioners, refrigerators and heat pumps produce a controlled heat transfer by the evaporation of a liquid refrigerant under appropriate pressure conditions in a heat exchanger to produce desired evaporator temperatures. The liquid refrigerant flowing through the heat exchanger removes heat from the medium being cooled and in this process is converted into a vapor at the same pressure and temperature. This vapor then flows into a compressor wherein its temperature and pressure are increased. The high pressure vapor then is conducted to a separate heat exchanger serving as a condenser wherein the gaseous refrigerant gives up heat to a heat transfer fluid in heat exchange relation therewith and changes state from a gas to a high pressure liquid. The high pressure liquid is then supplied to an expansion device which acts to reduce the pressure of the liquid refrigerant such that the liquid refrigerant may flow to the evaporator to begin the cycle again.
During the heating mode, a heat pump circuit utilizes an outdoor heat exchanger serving as an evaporator wherein the evaporator may be located in ambient air at a temperature below the freezing point of water. Thus, as this cold air is circulated over the heat exchanger, water vapor in the air is condensed and frozen on the surfaces of the heat exchanger. As the frost accumulates on the heat exchanger a layer of ice is built up between the portion of the heat exchanger carrying refrigerant and the air flowing thereover. This layer of ice acts as an insulating layer inhibiting the heat transfer in the coil between refrigerant and air. Additionally, on coils having fins to enhance heat transfer, the ice may serve to block the narrow air flow passageways between fins. This additional effect further serves to reduce heat transfer since lesser amounts of air will be circulated in heat exchanger relation with the refrigerant carrying conduits.
To efficiently operate a heat pump under conditions of relatively low outdoor ambient air temperatures it is necessary to provide apparatus for removing the accumulated frost. Many conventional methods are known such as supplying electric resistance heat, reversing the heat pump such that the evaporator becomes a condenser or other refrigerant circuiting techniques to direct hot gaseous refrigerant directly to the frosted heat exchanger.
Many of these defrost techniques utilize energy that is not effectively used for transferring heat energy to a space to be conditioned or to another end use served by the entire system. To reduce the amount of heat energy wasted or otherwise consumed in the defrost operation it is desirable to utilize a defrost system which adjusts the initiation and length of a defrost so that defrost lasts only for the time necessary to remove all frost, and subsequent to a defrost cycle reclaims a portion of the heat energy used in defrost to heat the space.
Different types of control systems have been utilized for adjusting the length of a defrost. A combination of a timer and a thermostat may be used to determine a period of a defrost. The thermostat periodically checks to see whether or not the evaporator temperature or a temperature dependent thereon is below a selected level, and if so acts to place the system in defrost for a period of time dependent on the timer. Other types of prior art defrost initiation systems have included measuring infrared radiation emitted from the fins of the refrigerant carrying coil, measuring the air pressure differentials of the air flow flowing through the heat exchanger, utilizing an electrical device placed on the fin whose characteristics change depending on the temperature of the device, optical-electrical methods and other methods involving the monitoring of various electrical parameters.
A disadvantage of the prior defrost modes is that they do not take into account changes in ambient temperature which can change the frost built-up on a coil between defrosts, nor do they attempt to reclaim the energy used in a defrost.
Thus, there is a clear need for a defrost system that adjusts the initiation of defrost in response to environmental conditions to optimize the length of a defrost, and also reclaims heat from the outdoor coil after the defrost cycle for use in heating the space being conditioned.